One of my favourite commentators on St. Paul is N.T. Wright, the Church of England's bishop of Durham. Wright wrote a series of books on Paul's letters - Paul For Everyone - that are engaging and easy to understand.

He has also penned various other books on the apostle to the gentiles. One is What St. Paul Really Said.

Another set of three much longer books on Christian origins includes sections on Paul's writings. The third volume, for example, is called The Resurrection of the Son of God and is 740 pages, including almost 200 pages on Paul's understanding of the resurrection.

Maybe you would want to try something shorter first.

The Navarre Bible: Letters of St. Paul includes the text of Paul's 13 letters (Revised Standard Version) and line-by-line commentaries written by scholars at the Opus Dei-run University of Navarre in Spain.

Those commentaries tap into Church fathers, such as St. John Chrysostom and St. Thomas Aquinas, who have written extensively on St. Paul as well as St. Josemaria Escriva, the founder of Opus Dei.

If you want something very meaty there is James Dunn's The Theology of Paul the Apostle. At 808 pages, it is daunting. Lots of the argument is aimed at academics, but lots can be of benefit to the average educated reader.